Beacon calibrated afc radar

ABSTRACT

In combination with a radar unit having automatic frequency control means for tuning its receiver accurately to its transmitter, means for calibrating the radar unit with respect to range by using a frequency shift reflector located a known distance from the radar unit and serving to provide echos at a different frequency than normal skin reflections, the radar unit being provided with a range gate to selectively pass substantially only the shifted reflections through the receiver and with switching means to transfer the automatic frequency control circuit to be responsive to the frequency of the shifted reflections rather than to the radar&#39;&#39;s transmitted frequency.

United States Patent PRF 24 YNCHRONIZER [72] Inventors Duane G.Fredericks 3,354,457 11/1967 Pfab et al 343/ 17.7

Lockport; Prima ry Examiner-Rodney D. Bennett, Jr. 2 A No g 'sg'Assistant Examiner-T. H. Tubbesing E p 1969 AttorneyAlexander and Dowelli [45] Patented Apr.6, 1971 '[73] Assignee Sierra Research Corp.

ABSTRACT: In combination with a radar unit having auto- [54] BEACONCALBRATED AFC RADAR matic frequency control means for tuning itsreceiver accu- 6 Claims lnrawing Fig rately to its transmitter, meansfor calibrating the radar unit with respect to range by using afrequency shift reflector located a known distance from the radar unitand serving to f CL G015 provide echos at a different frequency thannormal skin reflec- Fle'd of Search tions the radar unit being provideda range gate t0 electively pass substantially only the shiftedreflections through [56] References cued the receiver and with switchinmeans to transfer the auto- 8 UNITED STATES PATENTS matic frequencycontrol circuit to be responsive to the 3,263,228 7/ 1966 Abrahams et al343/ 17.7 frequency of the shifted reflections rather than to theradar's 3,343,168 9/1967 Fayram 343l17.7 transmitted frequency.

MHz 4(\ OSOLLATOR L9 1F AMP. oar.

RECIEVER 2 T R 6o Deny [15 200% '20 RANGE GATE "4o Il'RANSMITTER 42 FSRAFC AND SEARCH SWEEP Patented A ril s, 1971 3,573,826

' 75 MHz 0sc1LLAT0R 52 l2 '0 IF AMP DET MIXER 60 MH: AND R x DISPLAY l4200 f (20 RANGE GATE 40 /3o q 240 TRANsM|TTLR w \42 A AFC AND SEARCHSWEEP 26k 28 LocAL A OSCILLATOR A PRF '24 SYNCHRONIZER INVENTORS, DUANEG. FREDERICKS ATTORNEYS 1 BEACON CALIBRATED AFC RADAR DISCLOSURE Thisinvention relates to calibration means for radar units, and moreparticularly to the combination of an improved AFC radar unit with aremotely located frequency-shift reflector with which the automaticfrequency control (AFC) in the radar can be made cooperative so thatduring calibration the system can be made substantially nonresponsive toskin reflections returning at the frequency initially transmitted by theradar unit.

Especially in th case of fixed-position radar units of which a highdegree of accuracy is expected, such as tracking radars or missilelaunching site radars, it is important to have a reliable way ofchecking the range calibration and sensitivity of the radar so that theoperator is assured of efficient operation. Ordinary skin reflectionsfrom known objects in the environs are often used for this purpose butsuch reflections are rather unreliable because there is no way to besure that a target located, for example, at a range of 5 miles has notchanged or been augmented by the presence of a vehicle which is notusually there. Moreover, natural targets vary as to reflectivity fromtime to time with changes in surface dampness, foliage conditions,etc.", and hence cannot be relied upon for the purpose of checking radarsensitivity.

It is a principal object of this invention to provide a unique andreliable beacon target located at a known range and peculiarlycooperative with a radar unit of the type having AFC means normallymaintaining the radar receiver precisely tuned to its transmitterfrequency.

A further principal object of the invention is to provide in combinationwith the AFC radar unit a frequency-shifting beacon, reflector, or otherfrequency shifting and signal returning means, which responds toillumination at the radar frequency by returning an echo whose frequencyis shifted from the transmitted frequency of the radar by an amountplacing it outside of the normal passband of the radar receiver, so thatwhen the receiver is retuned to the beacon reflection frequency it willsuppress received echos arriving at the transmitted frequency of theradar. A suitable type of reflector is shown in Chisholm U.S. Pat. No.3,108,275.

Another principal object of the invention is to provide improvements insuch an AFC radar unit by which, during calibration mode of operation,the AFC means in the radar unit can be switched over to track thereceiver tuning to make it responsive to the shifted frequency of thebeacon reflector, and not to ordinary skin reflections.

Another major object of the invention is to provide means for making iteasier for the AFC to retune the receiver to the beacon responsefrequency when the calibration mode is selected by the operator. In theoperating embodiments of the present invention, the AFC has a sweep forsearch function which becomes operative when the AFC loses the inputsignal to which it has been tuning the receiver. lt sweeps through awide frequency range from one end toward the other and locks onto thefirst signal of suitable magnitude which it comes upon. Thus, when theoperator disconnects it from the transmitted frequency it beginssearching for another. it is now connected to acquire frequencies fromthe receiver, and it searches these frequencies always commencing fromthe same extreme end of the spectrum to which it is sensitive. This factcan be exploited by displacing the beacon frequency shift toward the endof the spectrum from which the AFC search always progresses so that theAFC will happen upon the shifted beacon frequency echo before it happensupon an echo occurring at the radar transmitted frequency, and willtherefore cease searching and tune the receiver to the beacon frequency.This likelihood is also greatly enhanced according to the presentinvention by providing an adjustable range gate for disabling the outputof the receiver except during reception of signals at the range of thebeacon reflector, thereby eliminating from the AFC all of the very largeamplitude skin reflections occurring especially at close range. Thisfunction can still further improved by placement of the beacon reflectorat a location devoid of other major skin reflection targets, whereby theopening of the aforesaid range gate at the range of the beacon reflectorwill pass to the AFC a signal whose frequency is predominantly composedof the beacon s shifted frequency and not of the originally transmittedfrequency.

Yet another important object of the invention is to provide pointtargets cooperating with an AFC radar system of the type employing twodifferent transmission frequencies, for instance in X-band and inKu-band, so that the radar can be aligned and calibrated at bothfrequencies using uniquely identifiable point targets at the same remotelocation, or perhaps using a single target operative at both of saidfrequencies.

A further object of the invention is to provide a simple and inexpensivecalibration system which can easily be incorporated into AFC radarsystems. An example of an AFC radar to which the present improvement canbe added is shown in Risely U.S. Pat. No. 3,412,335.

Other objects and advantages will become apparent during the followingdiscussion of the drawing, which is a block diagram showing anillustrative embodiment of the present invention.

Referring now to the drawing, the illustrative embodiment comprises aradar unit 10 including a transmitting and receiving antenna 12 coupledby a T/R box 14 to a transmitter 16 and a receiver mixer 18, the mixerfeeding an IF amplifier 20 whose output drives a detector and display 22of any suitable character. A synchronizer 24 modulates the transmitter16 and keys the receiver and display functions in a manner wellknown perse in the radar art. The receiver means also includes a local oscillator26 whose frequency can be adjusted by any one of a number ofarrangements, represented schematically by the wire 26The localoscillator frequency is controlled by output from an automatic frequencycontrol unit 28 having an input lead 28a which can be connected tosuitable means for providing a-sample frequency for the AFC unit 28 tolock onto. As in conventional radar systems having automatic frequencycontrol of the receiver, a sample of the transmitted pulse is taken fromthe wire 16a and passes through an attenuator 30 and via wire 30a to acrystal mixer 32 to which local oscillator frequency is also applied viathe wire 26b. The inputs on wire 30a and 26b to the crystal mixer 32provide an output on the wire 32a at the intermediate frequencyresulting from mixing the local oscillator and the transmitted pulse. Inthe present case the IF frequency is assumed to be 60 MHz. The crystalmixer box 32 is advantageously provided with sufficient amplification at60 MHz. to select an lF output on the wire 32a. This output passesthrough the-switch 34 which is shown in its normal position and the AFCunit 28 determines how close to 60 MHz. the output on wire 32 is. ltthen issues a frequency-control signal on the wire 26a to the localoscillator 26, and this control signal adjusts the local oscillator toprovide accurate 60 MHz. 1F on the wire 32a. In this way,conventionally, the radar maintains its receiver precisely tuned to thefrequency of the pulse being transmitted by its transmitter 16 andassures that the output from the mixer 18 into the lF amplifier 20 willoccur at 60 MHZ. for each reflection returning at the same frequency asthe transmitter 16' delivered to the antenna 12. Thus far, the presentdiscussion has been directed toward a conventional AFC radar of the typeshown in U.S. Pat. Nos. 2,425,013; 2,537,597; 2,798,946; or 2,881,321.It really does not matter whether the radar is pulse operated, or is aCW type, since AFC-can be performed in the manner described inconnection with either type of radar.

The novel improvements according to the present invention include theaddition of the switch 34, and the addition of a range gate system 40 inthe case of a pulse radar type of system. The range gate includes acontrol knob 42 by which the gate can be moved back and forth along thesweep of the display unit 22 in order to deliver to the AFC system onlysuch echos as occur at the range of the reflector 44.

The present improvements also include the frequency-shiftbe ing beaconreflector comprising a waveguide horn 44 which is flared at its outerend and is coupled to a short section of waveguide 46 which is closed atthe opposite end 48 and is of such a length as to be reflective at thefrequency at which the system is intended to operate. It is notnecessarily a require ment that the end 48 of the waveguide be closed,but only that it be terminated so as to provide a normally reflectivecondition for radar signals entering the horn 44. A short-circuitingmeans 50, comprising a diode in the present illustrative embodiment, isconnected across the waveguide in a position such that, when theshort-circuiting device 50 is conductive, the waveguide section 46 willno longer be reflective to as great an extent. By proper proportioningof the parts 46. 48 an 50 the short-circuiting device 50 can be made tomodulate the reflectivity of the device so as to effectively mix theincoming radar frequency with an oscillation provided by the oscillator52 and thereby provide reflections which are shifted from the frequencyof the radar transmitter 16 by an amount determined by the frequency ofthe oscillator 52. In this case, it is assumed that the modulatingfrequency of the oscillator 52 is 75 MHz., and that as a result of thismodulation the signals reflected from the reflector will comprise twosidebands each displaced from the transmitter frequency by 75 MHZ.

It is the purpose of the present invention to use the AFC unit 28 tomove. the local oscillator frequency on wire 26b to a new frequency atwhich the radar will be sensitive only to one of the reflected sidebandsfrom the horn 44 because the other frequencies will be outside of thepassband of the IF amplifier 20. The range gate 40 supplies a signalfrom the [F amplifier to the upper terminal of the switch 34, and thenceto the AFC unit 28 when the switch is in its upper position. However ina preferred embodiment, not all output from the llF amplifier 20 issupplied to the AFC unit, but only output which occurs at a moment intime corresponding with the range at which the frequency-shift reflectorhorn 44 is located. As indicated above, this range is adjustable byrotation of the knob 42 which controls the moment at which the rangegate 40 is opened to pass the signal on wire 20a to the upper terminalof switch 34. The range itself may be taken from the trailing edge of aSchmitt trigger output, the leading edge of which is pulsed on by outputfrom the synchronizer on wire 24a to correspond with the moment oftransmission of the radars main-bang pulse. This is a common type ofrange gating and will sulfice in the present instance to pass only theshifted-frequency echo from the horn 44 to the AFC unit 28, whichshifted signal then becomes the main input to the AFC unit. The AFC mayalso receive a certain amount of skin echo response, and perhaps somemagetron splash, but these spurious signals should be at a reducedamplitude as compared with the frequency-shifted signals from the horn44.

The signal taken from the IF amplifier 20 on the wire 20a is taken froman earlier stage in the amplifier so that subsequent tuned stages of theIF amplifier will not discriminate against the frequency-shiftedresponse which will in the initial instance be 75 Ml-llz. removed fromthe 60 IF frequency to which the amplifier 20 is tuned. it may also benecessary to amplify a signal from the amplifier on wire 200 using theseveral stages of amplification found in the AFC unit 28, which is apurchased item.

At any rate the AFC, being now supplied only with beaconshifted inputvia the switch 34, will lock onto this frequency and readjust the localoscillator 26 so as to apply such a frequency to the mixer 18 that a 60MHZ. lF signal will appear at the outputs of the IF amplifier 20whenever a beaconfrequency signal is received at the antenna 14.

In this way, the radar receiver is retuned to be sensitive only to thefrequency-shifted reflections from the horn 44, and not to ordinary skinreflections. It will stay tuned to the beacon reflections so long as theswitch 34 remains in the upper position. Conversely, if the operatorreturns the switch 34 to the lower position, the input to the AFC willthen comprise only a mixed component of the transmitter frequency whichwill provide a frequency different than 60 MHz. on the wire 32a. Inevery case, the AFC unit always seeks to return the signal supplied toits input 28a to 60 MHz. by readjusting the frequency of the localoscillator 26. Therefore, the AFC unit 28 will lock onto the onlyfrequency now being supplied to it, and change the local oscillatoruntil that frequency becomes 60 MHZ. The radar system is now retuned toreceive skin reflections, and will resume its normal operation.

In summary, the present invention provides means by which uniquelyidentifiable signals can be received and displayed from afrequency-shift beacon located at a known range with respect to theradar unit, and in which an AFC unit is used either to tune the receiverto the transmitted radar frequency or else to tune the receiver to thebeacon-shifted frequency, preferably also using a range gate toeliminate other signals which might possibly cause confusion of the AFCunit. The indicator unit 22 always displays the output of the main [Pamplifier 20, and a technician can easily determine by looking at thedisplay whether or not the AFC unit has in fact tuned the receiver tothe shifted beacon-response frequency, the display showing onlybeacon-return echos if the receiver has been tuned to it but showingnormal skin echos if it has not. In the event that the AFC has somehowlocked onto a skin echo instead of a frequency-shifted signal appearingon the wire 200, the technician need only flip the switch 34 back andforth once in order to dump whatever signal the AFC unit has locked ontoand start it making a new search to find the shifted frequency comingfrom the horn 44.

lt is assumed of course that the antenna 12 is pointing toward the horn44 when the switch 34 is moved to the upper position, since there willbe no shifted-frequency reflection to tune to otherwise. Obviously manyvariations of the present system are possible so long as the variousembodiments use a frequency-shifted return from a remote source such asa reflector or beacon and use an automatic frequency control system inthe radar for selectively changing the tuning of the radar receiver fromthe magnetron frequency to the shifted return frequency.

We claim:

1. In combination with a radar unit including transmitter, receiver andindicator means, and including automatic frequency control (AFC) meanshaving an input normally connected to sample the transmitted frequency,and the AFC means being operative to maintain the receiver meansaccurately tuned to the sampled frequency, means for use in calibratingthe radar unit, including:

a. means located at a known distance from the radar unit and operativeto shift its transmitted frequency, and to return a shifted-frequencysignal to its receiver means; and

b. switch means in the radar unit for switching the input of the AFCmeans from the transmitter means to the receiver means to sample thereturning shifted-frequency signal, whereby the AFC means will retunethe receiver means thereto.

2. In a combination as set forth in claim 1, range gate means in theradar unit connected between said switch means and the receiver meansand including means for adjusting the gate means to conduct from thereceiver means to the switch means only those signals corresponding intime to signals returning from a distance corresponding with the knownrange of the frequency shifting means with respect to the radar unit.

3. In a system as set forth in claim 1, wherein the AFC means operatesin the intermediate frequency range of the receiver means and isnormally coupled to the output of the transmitter means through an AFCmixer and is connected to tune a local oscillator which is coupled bothto a mixer in the receiver means and to the AFC mixer, said switch meansbeing connected to selectively couple the AFC input to sample the outputof either of said mixers.

4. In a system as set forth in claim 3, the frequency shift of the shiftmeans being greater than one-half of the band pass of the receiverintermediate frequency amplifier, and the switch means being coupled toa point early in said amplifier subsequent to the mixer.

5. In a system as set forth in claim 1, wherein the AFC means is of thetype which sweeps across its frequency range always in the samedirection in search of a signal to lock onto, such sweeping occurringautomatically when the AFC means loses the signal it was previouslylocked onto, the shift means including a modulating oscillator havingits frequency selected to provide a shifted sideband signal displacedfrom the transmitted frequency on the side which the AFC means firstsweeps across.

6. in a system as set forth in claim 1, said receiver means having anamplifier operating at an intermediate frequency, and said shift meanshaving means for shifting the frequency of reflections by an amountgreater than half the bandwidth of the intermediate amplifier, and saidindicator means being coupled to display echos passing through theintermediate amplifier, whereby when the AFC means tunes the receivermeans to the shifted-frequency signals, the display will includesubstantially only the latter signals.

1. In combination with a radar unit including transmitter, receiver and indicator means, and including automatic frequency control (AFC) means having an input normally connected to sample the transmitted frequency, and the AFC means being operative to maintain the receiver means accurately tuned to the sampled frequency, means for use in calibrating the radar unit, including: a. means located at a known distance from the radar unit and operative to shift its transmitted frequency, and to return a shifted-frequency signal to its receiver means; and b. switch means in the radar unit for switching the input of the AFC means from the transmitter means to the receiver means to sample the returning shifted-frequency signal, whereby the AFC means will retune the receiver means thereto.
 2. In a combination as set forth in claim 1, range gate means in the radar unit connected between said switch means and the receiver means and including means for adjusting the gate means to conduct from the receiver means to the switch means only those signals corresponding in time to signals returning from a distance corresponding with the known range of the frequency shifting means with respect to the radar unit.
 3. In a system as set forth in claim 1, wherein the AFC means operates in the intermediate frequency range of the receiver means and is normally coupled to the output of the transmitter means through an AFC mixer and is connected to tune a local oscillator which is coupled both to a mixer in the receiver means and to the AFC mixer, said switch means being connected to selectively couple the AFC input to sample the output of either of said mixers.
 4. In a system as set forth in claim 3, the frequency shift of the shift means being greater than one-half of the band pass of the receiver intermediate frequency amplifier, and the switch means being coupled to a point early in said amplifier subsequent to the mixer.
 5. In a system as set forth in claim 1, wherein the AFC means is of the type which sweeps across its frequency range always in the same direction in search of a signal to lock onto, such sweeping occurring automatically when the AFC means loses the signal it was previously locked onto, the shift means including a modulating oscillator having its frequency selected to provide a shifted sideband signal displaced from the transmitted frequency on the side which the AFC means first sweeps across.
 6. In a system as set forth in claim 1, said receiver means having an amplifier operating at an intermediate frequency, and said shift means having means for shifting the frequency of reflections by an amount greater than half the bandwidth of the intermediate amplifier, and said indicator means being coupled to display echos passing through the intermediate amplifier, whereby when the AFC means tunes the receiver means to the shifted-frequency signals, the display will include substantially only the latter signals. 